1. Field of the Invention
The present invention generally relates to a current source circuit, and particularly to the configuration of a novel current source circuit in which linearity of the characteristic can be kept in a wider range.
2. Prior Art
FIG. 1(a) shows an example of typical configuration of the conventional current source circuit.
As shown in the drawing, the circuit is constituted by an npn-type bipolar transistor Q.sub.21 in which the collector is connected to an external input terminal EX, the base is connected to a control voltage V.sub.0, and the emitter is connected to the ground GND through a resistor R.sub.0, so that and a collector-emitter current I changes in accordance with a variation of the control voltage V.sub.0.
FIG. 1(b) shows another example of the typical configuration of the conventional current source circuit.
As shown in the drawing, the circuit is constituted by a pair of npn-type bipolar transistors Q.sub.22 and Q.sub.23 and a resistor R.sub.0, the respective bases of which are connected to each other. In this example, in the transistor Q.sub.23, the collector and the base are shorted so as to make the transistor form a diode connection, the collector is connected to a control voltage V.sub.0 through the resistor R.sub.0, and the emitter is connected to the ground GND. In the transistor Q.sub.22, on the other hand, the collector is connected to an external input terminal EX, and the emitter is connected to the ground GND. In this current source circuit, the transistors Q.sub.22 and Q.sub.23 constitute a current mirror circuit, and the configuration is made such that a collector-emitter current I in the transistor Q.sub.22 changes in accordance with a variation of the control voltage V.sub.0. FIG. 2 is a graph showing a general current-voltage characteristic of a bipolar transistor to be used in such a current source circuit as described above.
As shown in the graph of FIG. 2 (prior art), although the current-voltage characteristic of the bipolar transistor fundamentally has linearity, the characteristic is non-linear particularly in a region where the current value is small. In the conventional current source circuit constituted by transistors having such a characteristic, therefore, there has been a problem in that a substantial dynamic range is narrow because when the output current I becomes small, the linearity of the control characteristic is lost.
To cope with the foregoing problem, in accordance with an input, a control voltage V can be supplied to the current source circuit to thereby compensate for the non-linearity of the characteristic. Such a voltage supply circuit is however generally large in size so that the occupied area in an integrated circuit increases and the power consumption is large. Further, generally, such a circuit is significantly influenced by the scattering of the element characteristics, and therefore the proposal does not provide an effective solution for the above problem in an actual case.